The present invention relates to a current limiter of an inverter, and more particularly, to a current limiter for a voltage/frequency (V/F) type inverter (DC-to-AC converter) where continuous and reliable operation is guaranteed by blocking the output of the inverter for a predetermined time when too much current is generated for a rotary speed control system of a motor adopting a V/F type inverter.
The speed of an induction motor is expressed as N=N.sub.s (1-s) where N.sub.s is synchronization speed defined as N.sub.s =120 f/p. Thus, one of three variables, i.e., slip (s), frequency (f) or the number of poles (p), should be varied to control the speed. Here, the most efficient method is to vary the frequency. Also, generally, a voltage is proportionately varied together with the frequency to maintain a proper torque and a high efficiency when the frequency is varied. This is called a V/F method where the voltage is output in direct proportion to the frequency corresponding to the intended speed. Nowadays, the speed of an induction motor is controlled by adopting this method. This method is widely used since this system is easily obtained and operated by a user.
The induction motor induces a magnetic flux between a stator and a rotor according to the variations of voltage and/or current in a winding of the stator. Thus, unlike the case of a DC motor or AC synchronous motor, the strength of the magnetic flux is not constant in an induction motor. The operational characteristics are therefore complex. Also, it is difficult to maintain a constant level of the magnetic flux. On the other hand, the strength of the magnetic flux is approximately proportional to the ratio (V/F) of the output voltage to frequency. Thus, when the frequency is constant, a constant magnetic flux is generated so that the same torque characteristic as that of a DC motor or AC synchronous motor can be obtained. That is, the output frequency is determined with respect to an intended speed of the motor, based on such motor parameters as the number of poles and the like. Hence, a voltage proportional to the output frequency is applied to the motor. However, since the voltage and frequency are constantly maintained, a pulse width modulation (PWM) type inverter for converting the constant voltage and frequency into a variable voltage and a variable frequency has been widely used. This PWM method for applying the output voltage by converting the strength of the output voltage into a pulse width corresponding thereto is necessary for preventing the over-heating of a power transistor which leads to diminished gain.
FIG. 1 is a model circuit diagram of a single phase circuit approximated in a normal operation state, in the conventional three-phase induction motor.
As shown in FIG. 1, an output current Ii flowing in the circuit by a stator voltage Vs applied to the motor is a function of a resistance R, an induction L and a slip S. Here, R.sub.s represents resistance of a stator, R.sub.L represents resistance of a rotator, L.sub.s 1 represents leakage induction of the stator, L.sub.r 1 represents leakage inductance of the rotator, M represents a mutual inductance, and S represents the slip. On the other hand, since slip S is varied according to the load state and a rotary speed, output current Ii cannot be freely controlled in the V/F method. However, the power devices used in the inverter, such as a power transistor, power metal oxide semiconductor field-effect transistor (MOSFET) or the like, are sensitive to the current value so that a predetermined current limitation is provided on-the output current. That is, when the output current is over the allowable range of the switching device according to the operation state or load state, the switching device is damaged. Therefore, in order to prevent such damage, the motor should be stopped by blocking the current supplied to the motor which is above a predetermined reference value.
However, many problems are also caused by blocking the current output. For example, a user may attempt to operate the motor when the motor stops. Also, to keep the motor from stopping due to the over-current generated during a sudden acceleration, the user has to manually adjust acceleration time through trial and error. In the V/F method, for protecting the motor from damage, since the operation of the inverter cannot be continuously operated due to the blocking of the over-current, there is difficulty in the operation of the peripheral systems of the induction motor. Further, the V/F method is complicated in practical application.